In nucleic acids (DNA, RNA) which are biological macromolecules, enormous amounts of genetic information essential for vital activities are recorded as sequences composed of combinations of only 4 bases. Such a nucleic acid allows self-replication using itself as a template by the action of DNA polymerase, and further undergoes processes of RNA polymerase-mediated transcription and ribosome-mediated translation to ensure the transmission of genetic information from DNA to DNA, from DNA to RNA, and/or from RNA to protein. It is exclusive base-pairing rules (A:T/U, G:C) that enable these replication and transmission events of genetic information. In addition, nucleic acids can form a variety of higher-order structures and hence exert various functions. By way of example, it is one of the indications that a large number of novel nucleic acids having aptamer and/or ribozyme functions have been generated by in vitro selection techniques.
However, unlike proteins which are composed of 20 types of amino acids, the chemical and physical diversity of nucleic acids is limited by the fact that there are only 4 bases (2 base pairs) in natural nucleic acids. For example, functional RNAs (e.g., tRNA, rRNA, mRNA) found in living bodies utilize various modified bases to stabilize their own structure and/or RNA-RNA and RNA-protein interactions. Thus, it will be very advantageous to expand the repertory of new bases (base pairs) in developing novel functional nucleic acids.
With the aim of further expansion of nucleic acid functions, attempts have been made to design nucleosides or nucleotides having unnatural bases. There are two possible approaches for introducing modified bases (or unnatural bases) into nucleic acids: 1) direct introduction by chemical synthesis; and 2) introduction catalyzed by nucleic acid polymerase enzymes. In the case of 1), there is a need to solve some problems associated with chemical synthesis, such as the stability of amidite units and the presence of protecting groups appropriate for base moieties. If these problems are solved, various unnatural bases can be introduced in a site-selective manner. However, the nucleic acids thus obtained are difficult to amplify and it is also difficult to synthesize long-chain nucleic acids. In the case of 2), if the enzymes recognize substrates to cause replication and transcription between artificial base pairs in a complementary manner, nucleic acids containing such artificial base pairs can be amplified and prepared. However, such substrates and base pairs (unnatural nucleitides) are still under development.